Methods and systems for OFDM using code division multiplexing

ABSTRACT

In some embodiments of the invention, OFDM symbols are transmitted as a plurality of clusters. A cluster includes a plurality of OFDM sub-carriers in frequency, over a plurality of OFDM symbol durations in time. Each cluster includes data as well as pilot information as a reference signal for channel estimation. In some embodiments, a plurality of clusters collectively occupy the available sub-carrier set in the frequency domain that is used for transmission. In some embodiments of the invention data and/or pilots are spread within each cluster using code division multiplexing (CDM). In some embodiments pilots and data are separated by distributing data on a particular number of the plurality of OFDM symbol durations and pilots on a remainder of the OFDM symbol durations. CDM spreading can be performed in time and/or frequency directions.

PRIORITY CLAIM

This application is a continuation of and claims the benefit of priorityfrom U.S. patent application Ser. No. 14/079,131, entitled “Methods andSystems for OFDM using Code Division Multiplexing” and filed on Nov. 13,2013 (now allowed), which is a continuation of and claims the benefit ofpriority from U.S. patent application Ser. No. 13/333,463, entitled“Methods and Systems for OFDM using Code Division Multiplexing” andfiled on Dec. 21, 2012 (issued as U.S. Pat. No. 8,588,276 on Nov. 19,2013), which is a continuation of and claims the benefit of priorityfrom U.S. patent application Ser. No. 11/910,091, entitled “Methods andSystems for OFDM using Code Division Multiplexing” and filed on May 15,2008 (issued as U.S. Pat. No. 8,111,763 on Feb. 7, 2012), which is aNational Stage of and claims the benefit of priority fromPCT/CA2006/000523, entitled “Methods and Systems for OFDM using CodeDivision Multiplexing” and filed on Mar. 30, 2006, which claims thebenefit of priority from two (2) U.S. Provisional Patent Application No.60/759,461, entitled “MIMO-OFDM Air Interface” and filed on Jan. 17,2006, and No. 60/666,548, entitled “MIMO-OFDM Air Interface” and filedon Mar. 30, 2005, all of which are fully incorporated herein byreference for all purposes and to the extent not inconsistent with thisapplication.

BACKGROUND

Field of the Application

The invention relates to the field of wireless communications, morespecifically to systems and methods for supporting Orthogonal FrequencyDivision Multiplexed (OFDM) symbol transmission.

Background of the Disclosure

Orthogonal frequency division multiplexing (OFDM) is a form ofmultiplexing that distributes data over a number of carriers that have avery precise spacing in the frequency domain. The precise spacing of thecarriers provides several benefits such as high spectral efficiency,resiliency to radio frequency interference and lower multi-pathdistortion. Due to its beneficial properties and superior performance inmulti-path fading wireless channels, OFDM has been identified as auseful technique in the area of high data-rate wireless communication,for example wireless metropolitan area networks (MAN). Wireless MAN arenetworks to be implemented over an air interface for fixed, portable,and mobile broadband access systems.

SUMMARY

In a first broad aspect of the invention, there is provided a method oftransmitting comprising: defining a first plurality of OFDM symbols tocontain a plurality of clusters, each cluster comprising a plurality ofsub-carriers over a plurality of OFDM symbol durations; spreading atleast one of data and pilots within each cluster using code divisionmultiplexing (CDM); transmitting the first plurality of OFDM symbolsfrom a first transmit antenna.

In some embodiments a first sub-set of the OFDM symbol durations containthe pilots and a second sub-set of the OFDM symbol durations contain thedata, the first sub-set being distinct from the second sub-set.

In some embodiments spreading at least one of data and pilots withineach cluster using CDM comprises spreading only data in a time directionover multiple OFDM symbol durations or a frequency direction overmultiple sub-carriers.

In some embodiments spreading at least one of data and pilots withineach cluster using CDM comprises spreading only pilots in a timedirection over multiple OFDM symbol durations or a frequency directionover multiple sub-carriers.

In some embodiments the method is performed simultaneously for at leasttwo transmit antennas, each of the at least two transmit antenna beingassigned a respective CDM spreading code.

In some embodiments the method further comprises: for each of aplurality of users, spreading the data for the user using a respectiveCDM code to produce a respective spread data; adding together the spreaddata for inclusion in the data locations of the plurality of OFDMsymbols.

In some embodiments spreading data within each cluster comprisesspreading data in a time direction over multiple OFDM symbols.

In some embodiments spreading data within each cluster comprisesspreading data in a frequency direction over multiple sub-carriers.

In some embodiments the method further comprises for each of at leastone additional antenna: defining a respective plurality of OFDM symbolsto contain a plurality of clusters, each cluster comprising a pluralityof sub-carriers over a plurality of OFDM symbol durations; spreading atleast one of data and pilots within each cluster using a respective codedivision multiplexing (CDM) code; transmitting the respective pluralityof OFDM symbols from the additional transmit antenna.

In some embodiments transmitting from the first antenna and the at leastone additional antenna comprises transmitting from at least twotransmitters each having at least one antenna.

In some embodiments transmitting from the first antenna and the at leastone additional antenna comprises transmitting from one transmitterhaving the first antenna and the at least one additional antenna.

In some embodiments the method further comprises: for each antenna,assigning pilots in each sub-carrier of at least one OFDM symbolduration in each of the plurality of clusters and nulls in eachsub-carrier of at least one other OFDM symbol duration in the pluralityof clusters.

In some embodiments transmitting from the first antenna and the at leastone additional antenna comprises transmitting from two antennas, themethod further comprising: for the first transmit antenna, assigningpilots in each sub-carrier of at least one OFDM symbol duration in theplurality of clusters and assigning nulls in each sub-carrier of atleast one other OFDM symbol duration in the plurality of clusters; andfor the at least one additional antenna, inserting pilots in eachsub-carrier of the at least one other OFDM symbol duration in theplurality of clusters and inserting nulls in each sub-carrier of the atleast one OFDM symbol duration in the plurality of clusters.

In some embodiments transmitting from the first antenna and the at leastone additional antenna comprises transmitting from two transmit antennaand the method further comprises: for the first antenna of the twotransmit antennas, for repeating first and second clusters of theplurality of clusters assigning pilots in each sub-carrier of at leastone OFDM symbol duration of the first cluster and assigning nulls ineach sub-carrier of the at least one OFDM symbol duration of the secondcluster; and for the at least one additional antenna of the two transmitantennas, for the repeating first and second clusters assigning pilotsin each sub-carrier of at least one OFDM symbol duration of the secondcluster and inserting nulls in each sub-carrier of the at least one OFDMsymbol duration of the first cluster.

In some embodiments transmitting from the first antenna and the at leastone additional antenna comprises transmitting from two transmit antenna,the method further comprising: for the first antenna of the two transmitantennas, for repeating first and second clusters of the plurality ofclusters assigning pilots in each even numbered sub-carrier of at leastone OFDM symbol duration of the first cluster and assigning nulls ineach odd numbered sub-carrier of the at least one OFDM symbol durationof the second cluster; and for the at least one additional antenna ofthe two transmit antennas, for the repeating first and second clustersassigning pilots in each even numbered sub-carrier of at least one OFDMsymbol duration of the second cluster and assigning nulls in each oddnumbered sub-carrier of the at least one OFDM symbol duration of thefirst cluster.

In some embodiments a subset of the plurality of clusters provides anaccess channel comprising a particular number of sub-carriers over aparticular number of OFDM symbol durations for communication between amobile terminal and a base station.

In some embodiments the subset of the plurality of clusters thatprovides the access channel are mapped to locations that are spread outwithin the combined OFDM sub-carrier set of the plurality of clusters.

In some embodiments the plurality of clusters that provide the accesschannel are contiguous within the combined OFDM sub-carrier set of theplurality of clusters.

In some embodiments pilots and data share an OFDM sub-carrier set of thecluster in at least one OFDM symbol duration.

In some embodiments a plurality of pilot groups are arranged in eachcluster, each pilot group comprising multiple pilot locations, andwherein and pilots are spread across pilot locations within each pilotgroup.

In some embodiments the method is performed for at least one user, eachof the at least one user being assigned a respective CDM spreading code.

In some embodiments the plurality of pilot locations in each pilot groupare localized in close proximity and the pilot groups are distributedwithin the cluster.

In some embodiments the pilot locations in each pilot group arecontiguous in at least one of time and frequency.

In some embodiments the plurality pilot locations in each group arescattered within the cluster.

In some embodiments there is provided a transmitter adapted to performthe methods of any of the above embodiments.

In some embodiments there is provided a receiver adapted to receive asignal transmitted in accordance with any of the above embodiments.

In some embodiments the receiver is further adapted to extract pilotsfrom the signal and to perform channel estimation by interpolating intime and/or frequency directions.

In some embodiments the receiver is further adapted to perform channelestimation by averaging pilots.

In some embodiments the receiver is further adapted to despread at leastone of pilots and data using at least one CDM spreading code assigned tothe receiver.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described withreference to the attached drawings in which:

FIG. 1 is a block diagram of a cellular communication system;

FIG. 2 is a block diagram of an example base station that might be usedto implement some embodiments of the present invention;

FIG. 3 is a block diagram of an example wireless terminal that might beused to implement some embodiments of the present invention;

FIG. 4 is a block diagram of a logical breakdown of an example OFDMtransmitter architecture that might be used to implement someembodiments of the present invention;

FIG. 5 is a block diagram of a logical breakdown of an example OFDMreceiver architecture that might be used to implement some embodimentsof the present invention;

FIG. 6A is a schematic diagram of an example cluster pattern for use intransmission of OFDM symbols;

FIG. 6B is a schematic diagram of an example of cluster patterns for usein transmission of OFDM symbols over two antennas, one cluster patternfor each antenna;

FIGS. 7A, 7B and 7D are schematic diagrams of examples of clusterpatterns used for transmission of OFDM symbols using code divisionmultiplexing (CDM) for transmission of data according some embodimentsof the invention;

FIG. 7C is a schematic diagram of six cluster pairs used fortransmission of OFDM symbols using CDM for transmission of dataaccording an embodiment of the invention;

FIGS. 8A, 8B, 8C, 8D and 8E are schematic diagrams of examples clusterpattern used for transmission of MIMO-OFDM symbols using CDM fortransmission of data according to some embodiments of the invention;

FIG. 9 is a schematic diagram of a cluster pattern used for transmissionof OFDM symbols including data and pilots for two antennas according toan embodiment of the invention;

FIG. 10 is a schematic diagram of a cluster pattern used fortransmission of OFDM symbols including data and pilots for four antennasaccording to an embodiment of the invention;

FIG. 11 is another schematic diagram of cluster pairs used fortransmission of OFDM symbols using CDM for transmission of pilots foruse with four different spreading codes according to an embodiment ofthe invention; and

FIG. 12 is a schematic diagram of cluster pairs used for transmission ofOFDM symbols using CDM for transmission of pilots for use with sixdifferent spreading codes according to an embodiment of the invention.

DETAILED DESCRIPTION

OFDM symbol transmission schemes are provided by embodiments of theinvention in which user content is code division modulated prior to OFDMmodulation.

In some embodiments of the invention, OFDM symbols are transmitted as aplurality of clusters. A cluster includes a plurality of OFDMsub-carriers in frequency, over a plurality of OFDM symbol durations intime. Each cluster includes data as well as pilot information as areference signal for channel estimation. In some embodiments, aplurality of clusters collectively occupy the available sub-carrier setin the frequency domain that is used for transmission.

FIG. 6A shows an example cluster pattern 600 employed for transmittingdata and pilots. The cluster pattern 600 is shown having a twodimensional appearance in which the horizontal direction is frequencyand the vertical direction is time. Each vertical column of circlesrepresents a single sub-carrier over time. Each horizontal row ofcircles represents a duration of a single OFDM symbol. In the example ofFIG. 6A, the cluster includes four contiguous sub-carriers crossing sixOFDM symbol durations. In the example of FIG. 6A, second and fifth OFDMsymbol durations are used for transmission of pilots and the remainingOFDM symbol durations are used for data. In cluster pattern 600, in thesecond and fifth OFDM symbol durations, each sub-carrier is used for apilot.

More generally, parameters such as the number of sub-carriers and OFDMsymbol durations in a cluster, the number of pilots used in a clusterand the location of the pilots in the cluster are all implementationspecific and may vary from the values of the parameters in the exampleof FIG. 6A.

In FIGS. 6A and 6B (described below) pilots and data are separated suchthat for N+M OFDM symbol durations that pilots occupy each sub-carrierof N OFDM symbol durations of the cluster pattern and data occupy eachsub-carrier of M OFDM symbol durations of the cluster pattern.Separation of the pilots and data in some embodiments enables pilotsequences occupying the sub-carriers in the cluster to have a reducedPAPR so as to improve a signal to noise ratio for channel estimation.Separation of the pilots and data in some embodiments enables a flexibledesign that can be used for both OFDM and single-carrier FDM. In someembodiments the separation of pilots and data also enables the pilotsand/or data to be spread over sub-carriers or OFDM symbol durationsusing code division multiplexing techniques.

In some embodiments, clusters are designed to support multiple inputmultiple output (MIMO) transmission with two or more transmit antennas.FIG. 6B shows an example of a pair of cluster patterns 610, 611 employedfor transmitting data and pilots for two transmit antennas using MIMO.In the example of FIG. 6B, second and fifth OFDM symbol durations areused for transmission of pilots and nulls and the remaining OFDM symboldurations are used for data. In the first cluster pattern 610, in thesecond OFDM symbol duration, first and third sub-carriers are used forpilots and the second and fourth sub-carriers contain nulls during whichnothing is transmitted. In the fifth OFDM symbol duration, the secondand fourth sub-carriers are used for pilots and the first and thirdsub-carriers contain nulls. In the second cluster 611, the positioningof the nulls and pilots in OFDM symbol durations is reversed compared tothe first cluster 610.

For the two examples shown in FIG. 6B, the pattern of the first cluster610 can be transmitted from a first antenna, and the pattern of thesecond cluster 611 can be transmitted from a second antenna. In such animplementation, the second antenna transmits nulls where the firstantenna transmits pilots and vice versa. This is useful to ensureaccurate reception of the pilots and accurate channel measurement.

More generally, in some embodiments where there are N antennas, pilotsfor the N antennas are transmitted during selected OFDM symbols, andeach antenna transmits its pilots while the remaining N−1 antennastransmit nulls.

More generally, parameters such as the number of sub-carriers and OFDMsymbol durations in a cluster, the use of pilots and nulls, the numberof pilots used in a cluster and the location of the pilots in thecluster are all implementation specific and may vary from the values ofthe parameters in the example of FIG. 6B.

In some embodiments multiple different cluster patterns are used forsupporting multiple users with a single antenna. In some embodimentsmultiple different cluster patterns are used for supporting a singleuser with multiple antennas, single users each with a single antennathat cooperate in sending transmissions or multiple users with multipleantennas. In some embodiments multi-carrier code division multiplexing(MC-CDM) is employed in assigning data and/or pilots in a cluster.MC-CDM is a multiplexing scheme in which a spreading code is used tospread data and/or pilots over multiple sub-carriers and/or OFDM symboldurations.

In some embodiments in a non-MIMO environment, a single cluster patternsimilar to one of those described for use in a MIMO environment withpilots and nulls is employed, and a power boost is applied the pilots.

In some embodiments, in a non-MIMO environment, for uplink transmissionbetween mobile terminals and a base station, mobile terminals with acell identifier having an odd number select a first cluster pattern of apair of cluster patterns similar to that used in a MIMO environment andthe terminals with a cell identifier having an even number select asecond cluster pattern of the pair of cluster patterns.

In some embodiments in a non-MIMO environment, when a single clusterpattern of a pair of cluster patterns similar to that used in a MIMOenvironment employing both pilots and nulls is used for transmission,the null locations for a logical channel that can be used fortransmission of additional information.

In some embodiments, a terminal may choose one of the pilot patternsaccording to instructions from a base station when cooperative MIMO isused. Cooperative MIMO can be implemented by two single antennatransmitters that use corresponding pilot patterns having oppositepositioning of pilots and nulls. With cooperative MIMO, two differenttransmitters transmit using the same frequency resources, and thereceiver separates these using MIMO processing techniques.

In some embodiments, in a MIMO environment, multiple antennas are eachassigned a different pilot pattern.

Basic access channels are defined using the clusters that make up aframe. In a particular example the BACH may be composed of a number I(for example I=3) of clusters. In some embodiments where each clusterhas 16 sub-carriers, each BACH is 16.times.I data sub-carriers. Moregenerally, the number of clusters in a BACH and the number of datasub-carriers in a BACH are implementation specific and are not limitedby the particular example.

In some embodiments, there are two types of BACH: diversity BACH andsub-band BACH. Diversity BACH and sub-band BACH are logicalsub-channelizations or mappings performed on the clusters in each BACH.In diversity BACH, the set of clusters that make up the BACH are mappedto locations that are spread out within the overall OFDM symbolsub-carrier set. In sub-band BACH, the set of clusters that make up theBACH are contiguous within the overall OFDM symbol sub-carrier set. Insome embodiments the two types of BACH co-exist in a same OFDM symbolsub-carrier set.

In a particular example, a transmission frame includes six OFDM symbols.The transmission frame is further divided into a plurality of clustersN. Each BACH in the transmission frame consists of three clusters.Therefore, the transmission frame includes N/3 BACH. Each of the N/3BACH can be one of two different formats: a first format includes Ldiversity groups and K sub-band groups, such that L+K=N/3.

The values are N, L and K are implementation specific parameters. Moregenerally, the number of OFDM symbols in a transmission frames can begreater than or less than six. Also, the number of clusters in a BACHcan be greater than or less than three.

In some embodiments, each sub-carrier of particular OFDM symboldurations in the cluster are used for pilots and each sub-carrier of theother OFDM symbol durations are used for data. This is true for theexamples of FIGS. 6A and 6B. In some embodiments, by separating pilotsand data in this manner different types of processing can be supportedfor pilots and for data.

In some embodiments, data for multiple users is transmitted using thesame sub-carrier frequencies by multiplexing the data from multipleusers using CDM techniques, and using pilots that are individuallyassigned to particular sub-carriers in the cluster. In some embodimentspilots are assigned on a per antenna basis. In some embodiments pilotsare assigned on a per user basis. In some embodiments pilots areassigned on a per antenna per user basis. In some embodiments CDMtechniques are used for multiplexing pilot information as well as data.In some embodiments CDM techniques are used for multiplexing pilotinformation but not data. CDM multiplexing for pilots will be describedin detail below.

In some embodiments multi-carrier code division multiplexing (MC-CDM)techniques are used for assigning data and pilots to reduce interferencevariation for OFDM transmission. In some implementations, using MC-CDMenables N users to access the same BACH. In some implementations, usingMC-CDM enables processing gain to improve coverage in the region of thetransmitter.

In some embodiments MC-CDM techniques are used for assigning data andpilots in up-link (UL) communication between a mobile terminal and abase station (BS). For such embodiments, multiple users can transmitusing the same frequency resource but with different spreading codes toseparate their data and/or to separate their pilots. Where a given useris a multi-antenna user, a respective different spreading code isassigned to each antenna for each of data and/or pilots.

In some embodiments MC-CDM techniques are used for assigning data andpilots for down-link (DL) communication between the BS and the mobileterminal. For such embodiments, transmissions to multiple users can bemade on a single antenna using the same frequency resource but withdifferent spreading codes to separate their data and/or to separatetheir pilots in which case the transmissions to the multiple users areadded together prior to transmission. Alternatively, transmissions tomultiple users can be made on multiple antennas using the same frequencyresource but with different spreading codes to separate their dataand/or to separate their pilots.

Where a given user is a multi-antenna user, a respective differentspreading code is assigned to each antenna for each of data and/orpilots.

In some embodiments user separation by spreading is based on spreadingcodes with a length of N, where N may be equal to the number of usersthat can occupy a frequency resource (for example, a cluster), althoughother values of N can be used. Given a set of M data symbols, afterspreading there will be M×N symbols. Where M data symbols might havebeen transmitted using a single (more generally L) clusters or basicaccess channels, after spreading M (more generally M×L) clusters orbasic access channels are needed.

In some embodiments, to enable efficient channel estimation, clustersare grouped together and allocated in groups. For example, in aparticular implementation, each group contains two clusters and isreferred to as a cluster pair. The examples below assume grouping incluster pairs, but it should be readily apparent how the same approachescould be applied to different sized groups.

FIG. 7A shows an example cluster pattern 700 for use with up to fourusers in a non-MIMO environment using MC-CDM according to an embodimentof the invention. The cluster pattern 700 includes a cluster pairconsisting of a first cluster 702 and a second cluster 704 that arecontiguous in the frequency direction.

In FIG. 7A code spreading occurs in the frequency direction. The data ofeach of up to four users is spread over the four sub-carriers in eachcluster 702,704 using a particular code assigned to that user, generallyindicated at 706. The particular code for a single user is shown andincludes code elements C1, C2, C3 and C4 that correspond to the foursub-carriers, respectively. In some implementations, a respectivecluster pattern like that of FIG. 7A is transmitted from each ofmultiple (four in the example) antennas (of one or multipletransmitters), each using a different spreading pattern, and eachcontaining only one of the four pilots. This would be appropriate forup-link use, for example for transmissions from multiple mobile stationsto a base station. In other implementations, the cluster represents thesum of content transmitted for four different users from a singletransmitter in which the data locations are summed for the four usersafter being spread differently prior to transmission. This would beappropriate for down-link use, for example for transmissions from a basestation to multiple mobile stations. For multiple single user antennas,in those sub-carrier positions, what is transmitted is effectivelysummed in the air during transmission. Pilots for each of up to fourusers are transmitted. These are labeled as “1” for a first user, “2”for a second user, “3” for a third user and “4” for a fourth user andare illustrated being transmitted in a second and a fifth OFDM symbolduration. When there are less than four users, each user may have morethan one pilot. For example, for two users, pilots 1 and 2 are for afirst user and pilots 3 and 4 are for a second user.

The pilots inserted for each user form a scattered pilot pattern thatcan be used to perform channel estimation, and from which channelestimates for other sub-carrier positions can be determined throughinterpolation. In some embodiments, the four pilots are arranged in adifferent order for each OFDM symbol duration that contains the pilotsin the cluster. In other embodiments, the four pilots are arranged in asame order for two or more OFDM symbol durations that contain the pilotsin the cluster.

For example for a spreading code length of four, four users may transmitover four BACH simultaneously, each user being assigned a differentspreading code. A spreading code length of four would also enable only asingle user to transmit over four BACH with a larger processing gain.Processing gain is a ratio of transmission bandwidth to informationbandwidth that helps to measure a performance advantage of codespreading over narrowband signals. A spreading code length of four wouldalso enable two users to transmit over four BACH with a largerprocessing gain than four users, but a smaller processing gain than oneuser.

FIG. 7B shows another example of a cluster pattern 710 for up to fourusers in which the cluster pattern 710 includes a cluster pairconsisting of a first cluster 712 and a second cluster 714 that arecontiguous in the frequency direction, generally indicated at 716. Inthe example of FIG. 7B, code spreading occurs in the time direction.Data of each user is spread over four OFDM symbol durations in eachsub-carrier of the cluster using a particular code assigned to thatuser. For each user the particular code includes code elements C1, C2,C3 and C4 that correspond to the four OFDM symbol durations,respectively. For multiple single user antennas, in those sub-carrierpositions, what is transmitted is a summed in the air duringtransmission. For multiple multi-user antennas, in those sub-carrierpositions, what is transmitted may be summed in the transmitter beforetransmission. Four pilots labeled as “1” for a first user, “2” for asecond user, “3” for a third user and “4” for a fourth user aretransmitted in first and sixth OFDM symbol durations. When there areless than four users, each user may have more than one pilot. Forexample, for two users, pilots 1 and 2 are for a first user and pilots 3and 4 are for a second user. In some embodiments, the four pilots arearranged in a different order for each OFDM symbol duration thatcontains the pilots in the cluster. In other embodiments, the fourpilots are arranged in a same order for two or more OFDM symboldurations that contain the pilots in the cluster.

In some implementations, a respective cluster pattern like that of FIG.7B is transmitted from each of multiple (four in the example) antennas(of one or multiple transmitters), each using a different spreadingpattern, and each containing only one of the four pilots. This would beappropriate for up-link use, for example for transmissions from multiplemobile stations to a base station. In other implementations, the clusterrepresents the sum of content transmitted for four different users froma single transmitter in which the data locations are summed for the fourusers after being spread differently prior to transmission. This wouldbe appropriate for down-link use, for example for transmissions from abase station to multiple mobile stations.

Furthermore, assignment of OFDM symbol durations in which eachsub-carrier of the respective OFDM symbol duration is used for pilottransmission is an implementation specific parameter.

If the clusters of FIGS. 7A and 7B are used for a single user having asingle spreading code on a single antenna, the processing gain, orspreading factor, is larger compared to when the clusters are used formultiple users with each user employing a different spreading code.Conversely, the greater the number of users, the smaller the processinggain.

In a particular example, six cluster pairs are used to support fourBACHs for four users. More specifically, from an available frequencybandwidth, six cluster pairs are collectively assigned to four users,with the four users each using all of the six cluster pairs and theircontent is respectively separated by using different CDM codes.

FIG. 7C shows an example of six cluster pairs 750-755 used to supportfour BACHs for four users.

The clusters of FIG. 7C employ the pattern of FIG. 7A. It is to beunderstood that the cluster pattern is an implementation specificparameter and reflects various embodiments described above. Moregenerally, the number of cluster pairs used to support a given numberchannels for a given of users is implementation specific. Once again,the FIG. 7C patterns can be used for the up-link or down-link asdescribed previously for the other examples.

FIGS. 7A-C show cluster patterns in which the spreading code spreadsdata of each of up to four users over the four sub-carriers in eachcluster. More generally, the number of sub-carriers over which data canbe spread for multiple users can be greater or less than four. FIG. 7Dshows an example of a cluster pattern 720 for use by each user in whichthe cluster pattern 720 includes a cluster pair consisting of a firstcluster 722 and a second cluster 724 that are contiguous in thefrequency direction. In this case, a cluster is eight sub-carriers overN+2 OFDM symbol durations two of which contain pilots and N of whichcontain data. The data of each user is spread over eight sub-carriers ineach cluster using spreading code elements C1 to C8. A longer spreadingcode results in a larger cluster. In this example, spreading occurs inthe frequency direction. The first and last OFDM symbols of the clusterpair 720 contain pilot locations for up to eight different users labeled“1” through “8” respectively; since there is a length eight spreadingcode, up to eight different users can be transmitted using the samecluster. Once again, the FIG. 7D pattern can be used for the up-link ordown-link as described previously for the other examples.

In some embodiments the spreading can occur in the time direction overmultiple OFDM symbols as opposed to spreading occurring in the frequencydirection as shown in FIG. 7D.

Furthermore, assignment of OFDM symbol durations in which eachsub-carrier of the OFDM symbol duration is used for pilot transmissionis an implementation specific parameter and is not limited to the firstand last OFDM symbol durations of the cluster as shown in FIG. 7D, butmay be assigned to any of the symbols durations in the cluster. Also, insome embodiments, the eight pilots are arranged in a different order foreach OFDM symbol duration that contains the pilots in the cluster. Inother embodiments, the eight pilots are arranged in a same order for twoor more OFDM symbol durations that contain the pilots in the cluster.

More generally, parameters such as the number of sub-carriers and OFDMsymbol durations in a cluster, the use of pilots and nulls, the numberof pilots used in a cluster and the location of the pilots in thecluster are all implementation specific and may vary from the values ofthe parameters in the examples of FIGS. 7A-7D.

In single user or multiple user MIMO implementations, cluster patternsare defined to include data locations and pilot locations. Each pilotlocation is turned on, or active, for one of the antennas such that thepilots do not interfere with each other. The data locations can be usedby all antennas. Code spreading is employed to separate the datatransmitted on the different antennas, and to separate the datatransmitted on one antenna. Various specific examples are given below.

In some embodiments a cluster pattern is formed by a group of twocluster pairs that are contiguous in frequency or consecutive in time. Arespective cluster pattern thus formed is transmitted from each of twotransmit antennas. More generally, a cluster pattern can be formed bycombining N groups of cluster pairs, and transmitting a respectivecluster having such a pattern over each of N transmit antennas, whereN>=2.

FIG. 8A shows an example of cluster patterns 800, 801 for transmissionby a first and second antennas respectively for one or multiple users ina MIMO environment using MC-CDMA in accordance with an embodiment of theinvention. The cluster pattern 800 includes a first cluster pair 810 anda second cluster pair 812 for consecutive transmission by a firstantenna. In each of the two cluster pairs 810,812 code spreading of thedata occurs in the frequency direction as generally indicated at 816. Inthe illustrated example, each data symbol is spread by a factor of four,and the content for four different users after spreading is combined foreach data location. In the first cluster pair 810 of the first clusterpattern 800, sub-carriers of a third OFDM symbol duration are dedicatedto pilot transmission and sub-carriers of a fourth OFDM symbol durationare left blank so as not to interfere with pilots transmitted on thesecond antenna. In the second cluster pair 812 of the first clusterpattern 800, sub-carriers of the third OFDM symbol duration arededicated to pilot transmission and the sub-carriers of the fourth OFDMsymbol duration are left blank so as not to interfere with pilotstransmitted on the second antenna. In some embodiments the pilots in thesecond cluster pair 812 are ordered differently than those of the firstcluster pair 810.

Similarly, in the second cluster pattern 801, the sub-carriers of thethird OFDM symbol duration are left blank so as and sub-carriers of thefourth OFDM symbol duration are dedicated to pilot transmission.

In the illustrated example, for the first antenna pattern 800, pilotsfor up to four different users are transmitted and are labeled “1”, “2”,“3” and “4” respectively. The pilots for a given user are scattered infrequency and time so as to allow interpolation of channel estimates. Asimilar arrangement of pilots for four users is shown in the secondcluster pattern 801. A total of up to eight users can supported withthis example. Fewer spreading codes can be used to increase thespreading gain of each user. In some embodiments, multiple spreadingcodes can be assigned to a given user.

More generally, the number of sub-carriers and OFDM symbol durations ina cluster are configurable for different size clusters. Also the numberof OFDM symbol durations employed for pilots and nulls in a cluster areconfigurable. In addition, any one or more OFDM symbol duration of acluster can be selected for assigning pilots and/or nulls to each of thesub-carriers in the selected OFDM symbol duration.

In some embodiments, the ordering of pilots is different for each OFDMsymbol duration that contains the pilots in the cluster. In otherembodiments, the ordering of pilots is the same for two or more OFDMsymbol durations that contain the pilots in the cluster.

FIG. 8B shows another example of cluster patterns 820,821 fortransmission by a first and second antennas respectively for one ormultiple users in a MIMO environment using MC-CDMA. Each cluster pattern820,821 includes first and second cluster pairs (830,832 in the firstcluster pair and 834,835 in the second cluster pair) that are contiguousin time. In the two cluster patterns 820,821, code spreading of the dataoccurs in the time direction as indicated by 836. In the illustratedexample, the location of the data and pilot sub-carriers is the same asthat described above with reference to FIG. 8A.

More generally, the number of sub-carriers and OFDM symbol durations ina cluster are configurable for different size clusters. Also the numberOFDM symbol durations employed for pilots and nulls in a cluster areconfigurable. In addition, any one or more OFDM symbol duration of acluster can be selected for assigning pilots and/or nulls to each of thesub-carriers in the selected OFDM symbol duration.

In some embodiments, the ordering of pilots is different for each OFDMsymbol duration that contains the pilots in the cluster. In otherembodiments, the ordering of pilots is the same for two or more OFDMsymbol durations that contain the pilots in the cluster.

FIG. 8C shows another example of a first and second cluster patterns850,851 for transmission by two antennas. Each cluster pattern 850,851includes first and second cluster pairs (852,853 in the first clusterpair and 855,856 in the second cluster pair) that are contiguous infrequency. In the two cluster patterns 850,851, code spreading of thedata occurs in the frequency direction as indicated by 854. In a firstcluster 856 of the first cluster pair 852 of the first cluster pattern850, sub-carriers of a second OFDM symbol duration and a fifth OFDMsymbol duration are dedicated to pilot transmission. In a second cluster857 of the first cluster pair 852 of the first cluster pattern 850,sub-carriers of the second OFDM symbol duration and the fifth OFDMsymbol duration are left blank. The pilots in the second cluster pair853 are ordered differently than those of the first cluster pair 852.The second cluster pair 853 of the first cluster pattern 850 issubstantially the same as the first cluster pair 852. In the secondcluster pattern 851 the arrangement of OFDM symbol durations assigned tothe pilots and nulls are reversed so that the pilots transmitted by thetwo antennas do not interfere with each other. The content of one or upto eight users can be transmitted as in previous examples.

More generally, the number of sub-carriers and OFDM symbol durations ina cluster are configurable for different size clusters. Also the numberof rows of OFDM symbol durations and employed for pilots and nulls in acluster are configurable. In addition, any one or more OFDM symbolduration of a cluster can be selected for assigning pilots to some ofthe sub-carrier and nulls to remaining sub-carriers in the selected OFDMsymbol duration.

In some embodiments, the ordering of pilots is different for each OFDMsymbol duration that contains the pilots in the cluster. In otherembodiments, the ordering of pilots is the same for two or more OFDMsymbol durations that contain the pilots in the cluster.

FIG. 8D shows another example of first and second cluster patterns860,861 for transmission by two antennas. FIG. 8D is similar to FIG. 8Cin that each cluster pattern 860,861 includes first and second clusterpairs (862,863 in the first cluster pair and 865,866 in the secondcluster pair) that are contiguous in frequency. However, in the twocluster pairs 862,863 of FIG. 8D, code spreading of the data occurs inthe time direction as indicated by 864. Also, assigned OFDM symboldurations in which pilots and nulls occur are different.

More generally, the number of sub-carriers and OFDM symbol durations ina cluster are configurable for different size clusters. Also the numberof rows of OFDM symbol durations and employed for pilots and nulls in acluster are configurable. In addition, any one or more OFDM symbolduration of a cluster can be selected for assigning pilots to some ofthe sub-carrier and nulls to remaining sub-carriers in the selected OFDMsymbol duration.

In some embodiments, the ordering of pilots is different for each OFDMsymbol duration that contains the pilots in the cluster. In otherembodiments, the ordering of pilots is the same for two or more OFDMsymbol durations that contain the pilots in the cluster.

FIG. 8E is yet another example of first and second cluster patterns870,871 for transmission by two antennas. Each cluster pattern 870,871includes two cluster pairs (872,873 in the first cluster pair and875,876 in the second cluster pair) that are contiguous in frequency. Inthis example the pilots are transmitted on sub-carriers in each of afirst OFDM symbol duration and a last OFDM symbol duration of thecluster and data is transmitted on the sub-carriers of OFDM symboldurations in-between the first and last OFDM symbol durations. Incontrast to the examples of FIGS. 8A to 8D where pilots of a givenantenna were inserted in groups of consecutive OFDM sub-carrierlocations, in the example of FIG. 8E pilots of a given antenna areinserted in alternating OFDM sub-carrier locations. In each cluster876,877,878,879 of the first cluster pattern 870, first and thirdsub-carriers are dedicated to pilot transmission and second and fourthsub-carriers are blank. The first and third sub-carriers of the firsttwo clusters 876,877 transmit first, second, third and fourth pilotslabeled as 1, 2, 3 and 4, respectively interspersed with nulls and thispattern is repeated in the second two clusters 878,879. In the secondcluster pattern 871 the arrangement of the pilots and nulls arereversed, such that there is space-time diversity for the pilotstransmitted at different times from different antenna. The content ofone or up to eight users can be transmitted as in previous examples withspreading in the time or frequency direction.

More generally, in the example of FIG. 8E, the pilots are assigned toeven sub-carriers and nulls are assigned to odd sub-carriers inclusters, or vice versa.

More generally, the number of sub-carriers and OFDM symbol durations ina cluster are configurable for different size clusters. Also the numberof rows of OFDM symbol durations and employed for pilots and nulls in acluster are configurable. In addition, any one or more OFDM symbolduration of a cluster can be selected for assigning pilots to some ofthe sub-carriers and nulls to remaining sub-carriers in the selected oneor more OFDM symbol duration.

In some embodiments, the ordering of pilots is different for each OFDMsymbol duration that contains the pilots in the cluster. In otherembodiments, the ordering of pilots is the same for two or more OFDMsymbol durations that contain the pilots in the cluster.

Furthermore, the assignment of OFDM symbol durations used for pilots asopposed to data is an implementation specific parameter.

In some embodiments, an arbitrarily defined scattered pilot clusterpattern is inserted for each antenna and/or each user. FIG. 9 shows aspecific example of this, where cluster patterns for two users are shownin a single cluster pair. The cluster pair includes first and secondclusters 901,902 that are contiguous in the frequency direction, eachcluster having four sub-carriers over ten OFDM symbol durations. In FIG.9, there are two pilot groups 910,912 each having six pilots. Morespecifically, the first pilot pattern 910 consists of a first pilot 910Alocated at a first OFDM symbol duration of a first sub-carrier in thefirst cluster 901, a second pilot 910B located at a second OFDM symbolduration of a second sub-carrier in a second cluster 902, a third pilot910C located at a fifth OFDM symbol duration of a third sub-carrier inthe first cluster 901, a fourth pilot 910D located at a sixth OFDMsymbol duration of a fourth sub-carrier in the second cluster 902, afifth pilot 910E located at a ninth OFDM symbol duration of the firstsub-carrier in the first cluster 901, and a pilot element 910F locatedat a tenth OFDM symbol duration of the second sub-carrier in the secondcluster 902. The second pilot group 912 consists of a first pilot 912Alocated at the second OFDM symbol duration of the first sub-carrier inthe first cluster 901, a second pilot 912B located at the first OFDMsymbol duration of the second sub-carrier in a second cluster 902, athird pilot 912C located at the sixth OFDM symbol duration of the thirdsub-carrier in the first cluster 901, a fourth pilot 912D located at thefifth OFDM symbol duration of the fourth sub-carrier in the secondcluster 902, a fifth pilot 912E located at the tenth OFDM symbolduration of the first sub-carrier in the first cluster 901, and a sixthpilot 912F located at the ninth OFDM symbol duration of the secondsub-carrier in the second cluster 902.

In a single antenna case, each of two users can be assigned one of thetwo cluster patterns. A first user transmits pilots in the locations ofthe first pilot group 910 and a second user transmits pilots in thelocations of the second pilot group 912.

The pilot pattern can be used for two antennas each assigned to oneuser. A first antenna transmits pilots in the locations of the firstpilot group 910 and a second antenna transmits pilots in the locationsof the second pilot group 912.

It is to be understood that the number of elements in each pilot groupis implementation specific and can be greater than or less than sixelements, as shown in FIG. 9. It is to be understood that the number ofpilot groups is implementation specific and can be greater than or lessthan two elements, as shown in FIG. 9.

In some embodiments the pilot locations in a pilot group are a group ofpilot locations that are contiguous in time and/or frequency. Moregenerally, the pilot locations of a pilot group can be a collection ofpilot locations each assigned to any particular sub-carrier of an OFDMsymbol duration.

FIG. 10 shows an assignment of pilots in a cluster pattern 1000 inaccordance with an embodiment of the invention. The cluster pattern 1000includes first and second clusters 1002,1004 that are contiguous in thefrequency direction, each cluster having four sub-carriers over ten OFDMsymbol durations. In FIG. 10, there are four pilot groups1010,1012,1014,1016 each having four elements. Each element within thecluster has a location defined by a OFDM symbol duration and asub-carrier.

In the illustrated example, in the cluster pattern 1000, a first pilotgroup 1010 is assigned to a first element 1010A located at a first OFDMsymbol duration of a first sub-carrier in the first cluster 1002, asecond element 1010B located at a second OFDM symbol duration of asecond sub-carrier in the second cluster 1004, a third element 1010Clocated at a seventh OFDM symbol duration of a third sub-carrier in thefirst cluster 1002 and a fourth element 1010D located at an eighth OFDMsymbol duration of a fourth sub-carrier in the second cluster 1004. Asecond pilot group 1012 is assigned to a first element 1012A located ata second OFDM symbol duration of the first sub-carrier in the firstcluster 1002, a second element 1012B located at the first OFDM symbolduration of the second sub-carrier in the second cluster 1004, a thirdelement 1012C located at the eighth OFDM symbol duration of the thirdsub-carrier in the first cluster 1002 and a fourth element 1012D locatedat the seventh OFDM symbol duration of the fourth sub-carrier in thesecond cluster 1004. A third pilot group 1014 is assigned to a firstelement 1014A located at a third OFDM symbol duration of the thirdsub-carrier in the first cluster 1002, a second element 1014B located ata fourth OFDM symbol duration of the fourth sub-carrier in the secondcluster 1004, a third element 1014C located at a ninth OFDM symbolduration of the first sub-carrier in the first cluster 1002 and a fourthelement 1014D located at a tenth OFDM symbol duration of the secondsub-carrier in the second cluster 1004. A fourth pilot grouping isassigned to a first element 1016A located at the fourth OFDM symbolduration of the third sub-carrier in the first cluster, a second element1016B located at the third OFDM symbol duration of the fourthsub-carrier in the second cluster, a third element 1016C located at thetenth OFDM symbol duration of the first sub-carrier in the first clusterand a fourth element 1016D located at the ninth OFDM symbol duration ofthe second sub-carrier in the second cluster.

In a single antenna case, four users are each assigned four pilots, onepilot to each pilot location in a respective pilot group.

In single user MIMO operation, four antennas each with a single user areassigned four pilots, one pilot to each pilot location in a respectivepilot group. In multi user MIMO operation, two antennas each with a twousers are assigned four pilots, one pilot to each pilot location in arespective pilot group

More generally, parameters such as the number of sub-carriers and OFDMsymbol durations in a cluster, the use of pilots and nulls, the numberof pilots used in a cluster and the location of the pilots in thecluster are all implementation specific and may vary from the values ofthe parameters in the examples of FIG. 10.

It is to be understood that the number of elements in each pilot groupis implementation specific and can be greater than or less than fourelements, as shown in FIG. 10. It is to be understood that the number ofpilot groups is implementation specific and can be greater than or lessthan four elements, as shown in FIG. 10.

In some embodiments the pilot locations in a pilot group are a group ofpilot locations that are contiguous in time and/or frequency. Moregenerally, the pilot locations of a pilot group can be a collection ofpilot locations each assigned to any particular sub-carrier of an OFDMsymbol duration.

In the examples described thus far, no CDM has been applied to thepilots. In some embodiments, the pilots are also scattered within thecluster using CDM techniques.

With CDM pilot spreading, pilots are organized into groups of N (whereN>=2), and code spreading is employed to allow pilot transmission tooccur on all of the pilots in each group for multiple users in a mannersimilar to that described in detail above for spreading of data. For agiven user, a single pilot is spread using a length N spreading sequenceto produce N pilots for the user. This is done for multiple users, withthe pilots of the multiple users being added together for transmission.

Channel estimates that are made on the basis of such spread pilots willreflect an average of channel conditions over the group of pilotlocations. Depending on whether spreading is done in the time directionor the frequency direction, the average will be over time or frequency.Several specific examples will now be given.

FIG. 11 shows an assignment of pilots in a cluster pattern 1100 inaccordance with an embodiment of the invention for use with fourdifferent spreading codes. The cluster pattern 1100 includes twoclusters 1101,1102 concatenated in the frequency direction, each clusterhaving four sub-carriers over ten OFDM symbol durations. In FIG. 11,each pilot group 1110,1112,1114,1116 includes four elements within thecluster. Each element has a location defined by a OFDM symbol durationand sub-carrier. In some embodiments pilots are spread over the fourlocations of each respective pilot group using a different spreadingcode. In some embodiments channel estimation is performed byinterpolating between pilot groups in one or two dimensions.

In the illustrated example, in the cluster pattern 1100, a first pilotgroup 1110 is assigned to the first cluster 1101 in which a firstelement 1110A is located at a first OFDM symbol duration of a firstsub-carrier, a second element 1110B is located at a second OFDM symbolduration of the first sub-carrier, a third element 1110C is located at athird OFDM symbol duration of a third sub-carrier and a fourth element1110D is located at a fourth OFDM symbol duration of a fourthsub-carrier. A second pilot group 1112 is assigned to the second cluster1102 in which a first element 1112A is located at the first OFDM symbolduration of a first sub-carrier, a second element 1112B is located atthe second OFDM symbol duration of the second sub-carrier, a thirdelement 1112C is located at the third OFDM symbol duration of a fourthsub-carrier and a fourth element 1112D is located at the fourth OFDMsymbol duration of the fourth sub-carrier. A third pilot group 1114 isassigned to the first cluster 1101 in which a first element 1114A islocated at a seventh OFDM symbol duration of the third sub-carrier, asecond element 1114B is located at an eighth OFDM symbol duration of thethird sub-carrier, a third element 1114C is located at a ninth OFDMsymbol duration of the first sub-carrier and a fourth element 1114D islocated at a tenth OFDM symbol duration of the first sub-carrier. Afourth pilot grouping is assigned to the second cluster 1101 in which afirst element 1116A is located at the seventh OFDM symbol duration ofthe fourth sub-carrier, a second element 1116B is located at the eighthOFDM symbol duration of the fourth sub-carrier, a third element 1116C islocated at the ninth OFDM symbol duration of the second sub-carrier anda fourth element 1116D is located at the tenth OFDM symbol duration ofthe second sub-carrier.

In the above-described example, with CDM pilot spreading, for a givenuser, four pilots are spread, one in each pilot group, each using alength four spreading sequence. The channel estimation can then beperformed by interpolating between the values of the pilot groups. Thisis particularly useful when the channel characteristics are known tovary frequently. In another embodiment, when the channel characteristicsare known to change more slowly, the four pilots for each user can bespread over one pilot location of each pilot group.

It is to be understood that the number of elements in each pilot groupis implementation specific and can be greater than or less than fourelements, as shown in FIG. 11. It is to be understood that the number ofpilot groups is implementation specific and can be greater than or lessthan four elements, as shown in FIG. 11.

Furthermore, parameters such as the number of sub-carriers and OFDMsymbol durations in a cluster, the use of pilots and nulls, and thelocation of the pilots in the cluster are all implementation specificand may vary from the values of the parameters in the examples of FIG.11.

In some embodiments the pilot locations in a pilot group are a group ofpilot locations that are contiguous in time and/or frequency. Moregenerally, the pilot locations of a pilot group can be a collection ofpilot locations each assigned to any particular sub-carrier of an OFDMsymbol duration.

FIG. 12 shows an assignment of pilots in a cluster pattern 1200 inaccordance with an embodiment of the invention for use with sixspreading codes. The cluster pattern 1200 includes two clusters1201,1202 concatenated in the frequency direction, each cluster havingfour sub-carriers over ten OFDM symbol durations. In FIG. 12, each pilotgroup 1210,1212,1214,1216 includes six elements within the clusterdefined by a OFDM symbol duration and sub-carrier. The pilots are spreadover the six locations using a different spreading code. In a firstcluster 1201 of the cluster pattern 1200, a first pilot grouping isassigned to the first three OFDM symbol durations in each of the firsttwo sub-carriers 1210A, 1210B, 1210C, 1210D, 1210E, 1210F. A secondpilot grouping is assigned to the last three OFDM symbol durations ineach of the first two sub-carriers 1214A, 1214B, 1214C, 1214D, 1214E,1214F. In a second cluster of the cluster pair, a third pilot groupingis assigned to the first three OFDM symbol durations in each of the lasttwo sub-carriers 1212A, 1212B, 1212C, 1212D, 1212E, 1212F. A fourthpilot grouping is assigned to the last three OFDM symbol durations ineach of the last two sub-carriers 1216A, 1216B, 1216C, 1216D, 1216E,1216F.

In a single antenna case, six users are each assigned a spreading code.Each of the six users employs its respective code to spread the pilotsignal over the six locations in each of the four pilot groups1210,1212,1214,1216. For example, a spreading code is used to spread afirst pilot signal for each user over the six pilot locations in pilotgroup 1210, a second pilot signal for each user over the six pilotlocations in pilot group 1212, a third pilot signal for each user overthe six pilot locations in pilot group 1214 and a fourth pilot signalfor each user over the six pilot locations in pilot group 1216.

In single user MIMO operation, the pilot pattern is employed for sixantennas each assigned to one user. Each of the six antennas has adifferent code to spread the pilot signal over the six pilot locationsof each pilot group. For example, a spreading code is used to spread afirst pilot signal for each user over the six pilot locations in pilotgroup 1210, a second pilot signal for each user over the six pilotlocations in pilot group 1212, a third pilot signal for each user overthe six pilot locations in pilot group 1214 and a fourth pilot signalfor each user over the six pilot locations in pilot group 1216.

In multi-user MIMO operation, the pilot pattern is employed for threeusers are each assigned to two antennas. In other multi-user MIMOembodiments, two users are each assigned to three antennas.

In the above-described example, with CDM pilot spreading, for a givenuser, four pilots are spread, one in each pilot group, each using alength four spreading sequence. The channel estimation can then beperformed by interpolating between the values of the pilot groups. Thisis particularly useful when the channel characteristics are known tovary frequently. In another embodiment, when the channel characteristicsare known to change more slowly, the four pilots for each user can bespread over one pilot location of each pilot group.

In some implementations the number of pilots locations in each pilotgroup is equal to or a multiple of a number of users and/or antennas.More generally, it is to be understood that the number of pilotlocations in each pilot group is implementation specific and can begreater than or less than six elements as shown in FIG. 12. It is to beunderstood that the number of pilot groups is implementation specificand can be greater than or less than six elements as shown in FIG. 12.

Parameters such as the number of sub-carriers and OFDM symbol durationsin a cluster, the use of pilots and nulls, and the location of thepilots in the cluster are all implementation specific and may vary fromthe values of the parameters in the examples of FIG. 12.

In some embodiments the pilot locations in a pilot group are a group ofpilot locations that are contiguous in time and/or frequency. Moregenerally, the pilot locations of a pilot group can be a collection ofpilot locations each assigned to any particular sub-carrier of an OFDMsymbol duration. If the pilot locations of a pilot group are localizedin close proximity and the pilot groups are distributed within thecluster, spreading may be done over the pilot locations in the group andchannel estimation performed by interpolating over the pilots groups intime and/or frequency directions. In another embodiment in which pilotlocations of pilot groups are localized in close proximity spreading maybe done over a pilot location in each pilot group and channel estimationis an average of the pilot location values. Examples are shown in FIGS.11 and 12.

If the pilot locations of a pilot group are distributed within thecluster, spreading may be done over a pilot location in each pilot groupto obtain a logical localized set of pilot locations and channelestimation is performed by interpolating over the logical localized setsof pilot locations in time and/or frequency directions. In anotherembodiment in which pilot locations of a pilot group are distributedwithin the cluster spreading may be done over a pilot location in eachpilot group and channel estimation is an average of the pilot locationvalues.

While FIGS. 9 and 10 were not described as having spreading over thepilot groups, it is to be understood that those cluster patterns couldemploy CDM techniques to spread pilots across all pilot locations ofeach pilot group or across a pilot location of each pilot group andperform channel estimation as described above.

A disadvantage of an increased number of pilots in a cluster is anincrease in the ratio of overhead to payload.

In some embodiments of the present invention systems and methods supporttransformed OFDM (T-OFDM) transmissions. T-OFDM is described in furtherdetail in PCT Patent Application No. PCT/CA2006/000464 filed Mar. 30,2006, titled “Method and system for Comdining OFDM and TransformedOFDM”, by Jianglei Ma et. al., which is assigned to the same assignee ofthe present application and hereby incorporated in its entirety. Notethat U.S. patent application Ser. No. 11/909,567 (now U.S. Pat. No.7,929,407) is the U.S. National Stage filing of PCT/CA2006/000464.

In some embodiments of the present invention systems and methods supportsingle user single, single antenna OFDM transmissions.

In some embodiments of the present invention systems and methods supportsingle user and multiple user MIMO and collaborative MIMO OFDMtransmissions.

For the purposes of providing context for embodiments of the inventionfor use in a communication system, FIG. 1 shows a base stationcontroller (BSC) 10 which controls wireless communications withinmultiple cells 12, which cells are served by corresponding base stations(BS) 14. In general, each base station 14 facilitates communicationsusing OFDM with mobile and/or wireless terminals 16, which are withinthe cell 12 associated with the corresponding base station 14. Themovement of the mobile terminals 16 in relation to the base stations 14results in significant fluctuation in channel conditions. Asillustrated, the base stations 14 and mobile terminals 16 may includemultiple antennas to provide spatial diversity for communications.

A high level overview of the mobile terminals 16 and base stations 14upon which aspects of the present invention are implemented is providedprior to delving into the structural and functional details of thepreferred embodiments. With reference to FIG. 2, a base station 14 isillustrated. The base station 14 generally includes a control system 20,a baseband processor 22, transmit circuitry 24, receive circuitry 26,multiple antennas 28, and a network interface 30. The receive circuitry26 receives radio frequency signals bearing information from one or moreremote transmitters provided by mobile terminals 16 (illustrated in FIG.1). A low noise amplifier and a filter (not shown) may cooperate toamplify and remove broadband interference from the signal forprocessing. Down-conversion and digitization circuitry (not shown) willthen down-convert the filtered, received signal to an intermediate orbaseband frequency signal, which is then digitized into one or moredigital streams.

The baseband processor 22 processes the digitized received signal toextract the information or data bits conveyed in the received signal.This processing typically comprises demodulation, decoding, and errorcorrection operations. As such, the baseband processor 22 is generallyimplemented in one or more digital signal processors (DSPs) orapplication-specific integrated circuits (ASICs). The receivedinformation is then sent across a wireless network via the networkinterface 30 or transmitted to another mobile terminal 16 serviced bythe base station 14.

On the transmit side, the baseband processor 22 receives digitized data,which may represent voice, data, or control information, from thenetwork interface 30 under the control of control system 20, and encodesthe data for transmission. The encoded data is output to the transmitcircuitry 24, where it is modulated by a carrier signal having a desiredtransmit frequency or frequencies. A power amplifier (not shown) willamplify the modulated carrier signal to a level appropriate fortransmission, and deliver the modulated carrier signal to the antennas28 through a matching network (not shown). Various modulation andprocessing techniques available to those skilled in the art are used forsignal transmission between the base station and the mobile terminal.

With reference to FIG. 3, a mobile terminal 16 configured according toone embodiment of the present invention is illustrated. Similarly to thebase station 14, the mobile terminal 16 will include a control system32, a baseband processor 34, transmit circuitry 36, receive circuitry38, multiple antennas 40, and user interface circuitry 42. The receivecircuitry 38 receives radio frequency signals bearing information fromone or more base stations 14. A low noise amplifier and a filter (notshown) may cooperate to amplify and remove broadband interference fromthe signal for processing. Down-conversion and digitization circuitry(not shown) will then down-convert the filtered, received signal to anintermediate or baseband frequency signal, which is then digitized intoone or more digital streams.

The baseband processor 34 processes the digitized received signal toextract the information or data bits conveyed in the received signal.This processing typically comprises demodulation, decoding, and errorcorrection operations. The baseband processor 34 is generallyimplemented in one or more digital signal processors (DSPs) andapplication specific integrated circuits (ASICs).

For transmission, the baseband processor 34 receives digitized data,which may represent voice, data, or control information, from thecontrol system 32, which it encodes for transmission. The encoded datais output to the transmit circuitry 36, where it is used by a modulatorto modulate a carrier signal that is at a desired transmit frequency orfrequencies. A power amplifier (not shown) will amplify the modulatedcarrier signal to a level appropriate for transmission, and deliver themodulated carrier signal to the antennas 40 through a matching network(not shown). Various modulation and processing techniques available tothose skilled in the art are used for signal transmission between themobile terminal and the base station.

In OFDM modulation, the transmission band is divided into multiple,orthogonal carrier waves. Each carrier wave is modulated according tothe digital data to be transmitted. Because OFDM divides thetransmission band into multiple carriers, the bandwidth per carrierdecreases and the modulation time per carrier increases. Since themultiple carriers are transmitted in parallel, the transmission rate forthe digital data, or symbols, on any given carrier is lower than when asingle carrier is used.

OFDM modulation utilizes the performance of an Inverse Fast FourierTransform (IFFT) on the information to be transmitted. For demodulation,the performance of a Fast Fourier Transform (FFT) on the received signalrecovers the transmitted information. In practice, the IFFT and FFT areprovided by digital signal processing carrying out an Inverse DiscreteFourier Transform (IDFT) and Discrete Fourier Transform (DFT),respectively. Accordingly, the characterizing feature of OFDM modulationis that orthogonal carrier waves are generated for multiple bands withina transmission channel. The modulated signals are digital signals havinga relatively low transmission rate and capable of staying within theirrespective bands. The individual carrier waves are not modulateddirectly by the digital signals. Instead, all carrier waves aremodulated at once by IFFT processing.

In operation, OFDM is preferably used for at least down-linktransmission from the base stations 14 to the mobile terminals 16. Eachbase station 14 is equipped with “n” transmit antennas 28, and eachmobile terminal 16 is equipped with “m” receive antennas 40. Notably,the respective antennas can be used for reception and transmission usingappropriate duplexers or switches and are so labeled only for clarity.

With reference to FIG. 4, a logical OFDM transmission architecture willbe described. Initially, the base station controller 10 will send datato be transmitted to various mobile terminals 16 to the base station 14.The base station 14 may use the channel quality indicators (CQIs)associated with the mobile terminals to schedule the data fortransmission as well as select appropriate coding and modulation fortransmitting the scheduled data. The CQIs may be directly from themobile terminals 16 or determined at the base station 14 based oninformation provided by the mobile terminals 16. In either case, the CQIfor each mobile terminal 16 is a function of the degree to which thechannel amplitude (or response) varies across the OFDM frequency band.

Scheduled data 44, which is a stream of bits, is scrambled in a mannerreducing the peak-to-average power ratio associated with the data usingdata scrambling logic 46. A cyclic redundancy check (CRC) for thescrambled data is determined and appended to the scrambled data usingCRC adding logic 48. Next, channel coding is performed using channelencoder logic 50 to effectively add redundancy to the data to facilitaterecovery and error correction at the mobile terminal 16. Again, thechannel coding for a particular mobile terminal 16 is based on the CQI.In some implementations, the channel encoder logic 50 uses known Turboencoding techniques. The encoded data is then processed by rate matchinglogic 52 to compensate for the data expansion associated with encoding.

Bit interleaver logic 54 systematically reorders the bits in the encodeddata to minimize the loss of consecutive data bits. The resultant databits are systematically mapped into corresponding symbols depending onthe chosen baseband modulation by mapping logic 56. Preferably,Quadrature Amplitude Modulation (QAM) or Quadrature Phase Shift Key(QPSK) modulation is used. The degree of modulation is preferably chosenbased on the CQI for the particular mobile terminal. The symbols may besystematically reordered to further bolster the immunity of thetransmitted signal to periodic data loss caused by frequency selectivefading using symbol interleaver logic 58.

At this point, groups of bits have been mapped into symbols representinglocations in an amplitude and phase constellation. When spatialdiversity is desired, blocks of symbols are then processed by space-timeblock code (STC) encoder logic 60, which modifies the symbols in afashion making the transmitted signals more resistant to interferenceand more readily decoded at a mobile terminal 16. The STC encoder logic60 will process the incoming symbols and provide “n” outputscorresponding to the number of transmit antennas 28 for the base station14. The control system 20 and/or baseband processor 22 as describedabove with respect to FIG. 2 will provide a mapping control signal tocontrol STC encoding. At this point, assume the symbols for the “n”outputs are representative of the data to be transmitted and capable ofbeing recovered by the mobile terminal 16.

For the present example, assume the base station 14 has two antennas 28(n=2) and the STC encoder logic 60 provides two output streams ofsymbols. Accordingly, each of the symbol streams output by the STCencoder logic 60 is sent to a corresponding IFFT processor 62,illustrated separately for ease of understanding. Those skilled in theart will recognize that one or more processors may be used to providesuch digital signal processing, alone or in combination with otherprocessing described herein. The IFFT processors 62 will preferablyoperate on the respective symbols to provide an inverse FourierTransform. The output of the IFFT processors 62 provides symbols in thetime domain. The time domain symbols are grouped into frames, which areassociated with a prefix by prefix insertion logic 64. Each of theresultant signals is up-converted in the digital domain to anintermediate frequency and converted to an analog signal via thecorresponding digital up-conversion (DUC) and digital-to-analog (D/A)conversion circuitry 66. The resultant (analog) signals are thensimultaneously modulated at the desired RF frequency, amplified, andtransmitted via the RF circuitry 68 and antennas 28. Notably, pilotsignals known by the intended mobile terminal 16 are scattered among thesub-carriers. The mobile terminal 16, which is discussed in detailbelow, will use the pilot signals for channel estimation.

Reference is now made to FIG. 5 to illustrate reception of thetransmitted signals by a mobile terminal 16. Upon arrival of thetransmitted signals at each of the antennas 40 of the mobile terminal16, the respective signals are demodulated and amplified bycorresponding RF circuitry 70. For the sake of conciseness and clarity,only one of the two receive paths is described and illustrated indetail. Analog-to-digital (A/D) converter and down-conversion circuitry72 digitizes and down-converts the analog signal for digital processing.The resultant digitized signal may be used by automatic gain controlcircuitry (AGC) 74 to control the gain of the amplifiers in the RFcircuitry 70 based on the received signal level.

Initially, the digitized signal is provided to synchronization logic 76,which includes coarse synchronization logic 78, which buffers severalOFDM symbols and calculates an auto-correlation between the twosuccessive OFDM symbols. A resultant time index corresponding to themaximum of the correlation result determines a fine synchronizationsearch window, which is used by fine synchronization logic 80 todetermine a precise framing starting position based on the headers. Theoutput of the fine synchronization logic 80 facilitates frameacquisition by frame alignment logic 84. Proper framing alignment isimportant so that subsequent FFT processing provides an accurateconversion from the time domain to the frequency domain. The finesynchronization algorithm is based on the correlation between thereceived pilot signals carried by the headers and a local copy of theknown pilot data. Once frame alignment acquisition occurs, the prefix ofthe OFDM symbol is removed with prefix removal logic 86 and resultantsamples are sent to frequency offset correction logic 88, whichcompensates for the system frequency offset caused by the unmatchedlocal oscillators in the transmitter and the receiver. Preferably, thesynchronization logic 76 includes frequency offset and clock estimationlogic 82, which is based on the headers to help estimate such effects onthe transmitted signal and provide those estimations to the correctionlogic 88 to properly process OFDM symbols.

At this point, the OFDM symbols in the time domain are ready forconversion to the frequency domain using FFT processing logic 90. Theresults are frequency domain symbols, which are sent to processing logic92. The processing logic 92 extracts the scattered pilot signal usingscattered pilot extraction logic 94, determines a channel estimate basedon the extracted pilot signal using channel estimation logic 96, andprovides channel responses for all sub-carriers using channelreconstruction logic 98. In order to determine a channel response foreach of the sub-carriers, the pilot signal is essentially multiple pilotsymbols that are scattered among the data symbols throughout the OFDMsub-carriers in a known pattern in both time and frequency. Examples ofscattering of pilot symbols among available sub-carriers over a giventime and frequency plot in an OFDM environment are found in PCT PatentApplication No. PCT/CA2005/000387 filed Mar. 15, 2005 assigned to thesame assignee of the present application. Continuing with FIG. 5, theprocessing logic compares the received pilot symbols with the pilotsymbols that are expected in certain sub-carriers at certain times todetermine a channel response for the sub-carriers in which pilot symbolswere transmitted. The results are interpolated to estimate a channelresponse for most, if not all, of the remaining sub-carriers for whichpilot symbols were not provided. The actual and interpolated channelresponses are used to estimate an overall channel response, whichincludes the channel responses for most, if not all, of the sub-carriersin the OFDM channel.

The frequency domain symbols and channel reconstruction information,which are derived from the channel responses for each receive path areprovided to an STC decoder 100, which provides STC decoding on bothreceived paths to recover the transmitted symbols. The channelreconstruction information provides equalization information to the STCdecoder 100 sufficient to remove the effects of the transmission channelwhen processing the respective frequency domain symbols.

The recovered symbols are placed back in order using symbolde-interleaver logic 102, which corresponds to the symbol interleaverlogic 58 of the transmitter. The de-interleaved symbols are thendemodulated or de-mapped to a corresponding bitstream using de-mappinglogic 104. The bits are then de-interleaved using bit de-interleaverlogic 106, which corresponds to the bit interleaver logic 54 of thetransmitter architecture. The de-interleaved bits are then processed byrate de-matching logic 108 and presented to channel decoder logic 110 torecover the initially scrambled data and the CRC checksum. Accordingly,CRC logic 112 removes the CRC checksum, checks the scrambled data intraditional fashion, and provides it to the de-scrambling logic 114 forde-scrambling using the known base station de-scrambling code to recoverthe originally transmitted data 116.

In parallel to recovering the data 116, a CQI, or at least informationsufficient to create a CQI at the base station 14, is determined andtransmitted to the base station 14. As noted above, the CQI may be afunction of the carrier-to-interference ratio (CR), as well as thedegree to which the channel response varies across the varioussub-carriers in the OFDM frequency band. The channel gain for eachsub-carrier in the OFDM frequency band being used to transmitinformation is compared relative to one another to determine the degreeto which the channel gain varies across the OFDM frequency band.Although numerous techniques are available to measure the degree ofvariation, one technique is to calculate the standard deviation of thechannel gain for each sub-carrier throughout the OFDM frequency bandbeing used to transmit data.

In some embodiments a receiver is adapted to receive a signaltransmitted in accordance with embodiments of the invention describedabove in which pilots and/or data are spread using CDM spreadingtechniques.

In some embodiments the receiver is further adapted to extract pilotsfrom the signal and to perform channel estimation by interpolating intime and/or frequency directions. In some embodiments the receiver isfurther adapted to perform channel estimation by averaging pilots. Insome embodiments the receiver is further adapted to despread at leastone of pilots and data using at least one CDM spreading code assigned tothe receiver.

FIGS. 1 to 5 each provide a specific example of a communication systemor elements of a communication system that could be used to implementembodiments of the invention. It is to be understood that embodiments ofthe invention can be implemented with communications systems havingarchitectures that are different than the specific example, but thatoperate in a manner consistent with the implementation of theembodiments as described herein.

Numerous modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

What is claimed is:
 1. A mobile terminal comprising: one or moreantennas configured to receive a first signal from a wireless channel,wherein the first signal includes a cluster, wherein the clustercorresponds to a plurality of subcarriers in frequency and a pluralityof transmit symbol intervals in time; and a receiver configured to:recover first pilot symbols from the cluster by applying a firstcode-division multiplexing (CDM) code to a first subset of locationswithin the cluster; and generate channel estimates for the wirelesschannel based on the first pilot symbols.
 2. The mobile terminal ofclaim 1, wherein the receiver is further configured to recover payloaddata from the cluster, using the channel estimates.
 3. The mobileterminal of claim 2, wherein the receiver is further configured tointerpolate the channel estimates in time and/or frequency to determineinterpolated channel estimates, wherein the interpolated channelestimates are usable to recover said payload data from a second subsetof locations within the cluster, wherein the first subset and secondsubset are non-overlapping.
 4. The mobile terminal of claim 1, whereinthe cluster is a superposition of cluster instances, wherein each of thecluster instances includes: (a) a corresponding set of code-spreadedpilot signals within the first subset of locations of the clusterinstance, wherein the locations of the first subset are common to allthe cluster instances; and (b) payload data within a second subset oflocations of the cluster instance, wherein at least a portion of thepayload data in a first of the cluster instances is intended for saidmobile terminal, wherein the locations of the second subset are commonto all cluster instances, wherein the first subset and the second subsetare non-overlapping.
 5. The mobile station of claim 4, wherein the firstsignal is an orthogonal frequency-divisional multiplexing (OFDM) signal.6. The mobile station of claim 1, wherein the one or more antennascomprise a plurality of antennas, wherein the antennas are configured tocapture respective antenna signals, wherein the receiver is furtherconfigured to generate the first signal by decoding the antenna signalsaccording to a space-time transmit code.
 7. The mobile station of claim1, wherein said first subset of locations comprises a plurality ofcontiguous groups, wherein the contiguous groups are separated from oneanother within the cluster.
 8. The mobile station of claim 7, whereinthe plurality of contiguous groups are distributed within the cluster inboth time and frequency.
 9. A method for operating a mobile terminal,the method comprising: receiving a first signal from a wireless channelvia one or more antennas, wherein the first signal includes a cluster,wherein the cluster corresponds to a plurality of subcarriers infrequency and a plurality of transmit symbol intervals in time; andrecovering first pilot symbols from the cluster by applying a firstcode-division multiplexing (CDM) code to a first subset of locationswithin the cluster; and generating channel estimates for the wirelesschannel based on the first pilot symbols.
 10. The method of claim 9,further comprising: recovering payload data from the cluster, using thechannel estimates.
 11. The method of claim 10, further comprising:interpolating the channel estimates in time and/or frequency todetermine interpolated channel estimates, wherein the interpolatedchannel estimates are usable to recover said payload data from a secondsubset of locations within the cluster, wherein the first subset andsecond subset are non-overlapping.
 12. The method of claim 9, whereinthe cluster is a superposition of cluster instances, wherein each of thecluster instances includes: (a) a corresponding set of code-spreadedpilot signals within the first subset of locations of the clusterinstance, wherein the locations of the first subset are common to allthe cluster instances; and (b) payload data within a second subset oflocations of the cluster instance, wherein at least a portion of thepayload data in a first of the cluster instances is intended for saidmobile terminal, wherein the locations of the second subset are commonto all cluster instances, wherein the first subset and the second subsetare non-overlapping.
 13. The method of claim 12, wherein the firstsignal is an orthogonal frequency-divisional multiplexing (OFDM) signal.14. The method of claim 9, wherein the one or more antennas comprise aplurality of antennas, wherein the antennas are configured to capturerespective antenna signals, the method further comprising: generatingthe first signal by decoding the antenna signals according to aspace-time transmit code.
 15. The method of claim 9, wherein said firstsubset of locations comprises a plurality of contiguous groups, whereinthe contiguous groups are separated from one another within the cluster.16. The method of claim 15, wherein the plurality of contiguous groupsare distributed within the cluster in both time and frequency.
 17. Anon-transitory memory medium storing program instructions which arereadable by a processor of a mobile terminal, wherein the programinstructions, when executed by the processor, cause the processor toimplement: receiving a first signal from a wireless channel via one ormore antennas, wherein the first signal includes a cluster, wherein thecluster corresponds to a plurality of subcarriers in frequency and aplurality of transmit symbol intervals in time; and recovering firstpilot symbols from the cluster by applying a first code-divisionmultiplexing (CDM) code to a first subset of locations within thecluster; and generating channel estimates for the wireless channel basedon the first pilot symbols.
 18. The memory medium of claim 17, furthercomprising: recovering payload data from the cluster, using the channelestimates.
 19. The memory medium of claim 17, wherein the cluster is asuperposition of cluster instances, wherein each of the clusterinstances includes: (a) a corresponding set of code-spreaded pilotsignals within the first subset of locations of the cluster instance,wherein the locations of the first subset are common to all the clusterinstances; and (b) payload data within a second subset of locations ofthe cluster instance, wherein at least a portion of the payload data ina first of the cluster instances is intended for said mobile terminal,wherein the locations of the second subset are common to all clusterinstances, wherein the first subset and the second subset arenon-overlapping.
 20. The memory medium of claim 17, wherein said firstsubset of locations comprises a plurality of contiguous groups, whereinthe contiguous groups are separated from one another within the cluster.